When studying the interaction between the ultra high-voltage direct current(UHVDC) system and the AC system, the accurate electrical quantity changes of DC fault are important. In order to accurately simulate and analyze the characteristics of fault changes in UHVDC transmission systems, on the PSCAD/EMTDC simulation platform, taking Binkin DC transmission system as an example, a 800 kV, 8 000 MW bipolar UHVDC transmission system model is built. This model mainly details the reactive power compensation equipment, filtering device, DC line and DC control system in the DC transmission system. For the transmission and reception AC systems, the Thevenin equivalents of the actual AC systems on both sides are made at the converter bus, and a practical system transmission model with a DC system as the main and a simplified AC system is built. The commutation failure of typical faults in the DC system is simulated and analyzed. The main electrical changes in UHVDC system before and after the failure of commutation failure are analyzed. The feasibility of the model is verified by comparing with the waveform of the actual fault recorder. The changes of active power and reactive power flowing into the receiving AC system on the inverter-side converter bus are emphasized. The research conclusion provides technical support for analyzing the impact of UHVDC commutation failure faults on the actual AC system.